Night Photography Resource

Sometimes conditions and the limits imposed by cameras will force you to choose to use a high ISO speed. There is nothing wrong with a high ISO speed, but some people do not like the additional grain that this adds to their picture. Image stacking, a technique from astronomy, can help improve your high ISO pictures by reducing noise.

Overview

Noise in pictures appears as random specks of light superimposed over your picture. Since the contents of the picture are non-random (assuming you have no movement in your picture), you can take multiple pictures and then blend them together so the random noise is canceled out.

Why Use This Method?

There are several noise reduction programs and tools available which perform similar work on a single photograph. The main drawback to using the other tools is that they tend to cause blurring when they remove noise. This technique is preferable since it maintains the sharpness of the original photograph.

How to

1. Using a tripod, take a series of pictures of the same scene using the same settings
2. Import the pictures into your favorite image editing program, each as a separate layer
3. Set the opacity of the bottom layer to 100%, the next lowest layer to 50%, the next lowest layer to 25% and so forth; halving the opacity for each subsequent layer
4. Flatten the image into a single layer and save normally

Examples

On the left is a picture taken at a high ISO speed without any compensation, and on the right is a composite made from stacking six high ISO speed pictures together. Notice that the picture on the right has reduced grain but still maintains its sharpness.

Ever had a roll of low sensitivity film (eg. ISO 200) and wish it was more sensitive (eg. ISO 800)?

Push processing allows you to increase the sensitivity of film by telling your camera that you are using a faster speed of film than you are, then developing the film at the new speed rating. This has the effect of underexposing the film in the camera and then overdeveloping it, so the overall effect is a properly exposed picture. This allows you to take pictures in darker environments without sacrificing your other camera settings, but it does cause higher contrast, increased grain and lower resolution.

To push film; set your camera to a film speed one or two stops faster than the film you are using and take pictures normally. then lengthen the development time to compensate. Some cameras detect the film speed automatically, in which case you would want to underexpose all the pictures by one or two stops to get this effect.

Film speed (referred to as ISO speed in digital cameras) is often the first choice you make when you take pictures. The type of film you use affects how sensitive it is to light.

The film speed ISO numbers indicate how sensitive the film is. Each time the numbers double (going from ISO 400 to ISO 800) the film is becoming twice as sensitive to light. Each time the numbers half (going from ISO 200 to ISO 100) the film is becoming half as sensitive to light.

Higher ISO numbers (800, 1600, 3200) are highly sensitive to light and require shorter exposure times while the lower ISO numbers (25, 50, 100) are not very sensitive and require longer exposure times.

Now you might think that it would be best to shoot with the fastest film you can buy since this will result in shorter exposures. The problem is that the higher your film speed is, the more grain (or noise in digital cameras) is in your picture. Low film speeds take very clear pictures, but require either long exposure times or bright conditions. See the attached file for an example of this. Photography is all about trade offs.